This tutorial shows how to share folder between virtualbox Ubuntu host and Windows guest,or Windows host and Ubuntu guest.
First of all,make sure you have installed guest additions(from virtual machine windows,click to Machines->Install guest additions).
Ubuntu host and Windows/Vista guest:
1.First,create the shared folder in ubuntu host,assuming the folder is /home/user_name/vbshare:

mkdir ~/vbshare

2.Boot up Windows guest and go to Devices->Shared Folders
virtualbox_share
Click the button at right to create a share.
In Folder Path navigate to the folder you created in step one and select it.Check “Make Permanent”.
virtualbox_share2
3.For Windows guest,navigate to Start->Run and type:

cmd

In dos console,use this command:

net use t: \\vboxsvr\vbshare

For Vista guest,open your Windows Explorer, click on the Map Network Drive.Type following in Folder area and check “Reconnect at logon”.

\\vboxsvr\vbshare

4.Open My computer,you will see the shared folder:
virtualbox_share3

Windows Host and Ubuntu guest:
1.Create an folder,assuming it is:

c:/vbshare

2.Same to the step 2 above.
3.In ubuntu guest,open terminal(Applications->Accessories->Terminal).Using following command mount the folder:

sudo mkdir /media/vb_share
sudo mount -t vboxsf vbshare /media/vb_share